The present invention relates to a press section of a paper making machine in which water is removed from a paper web formed in a preceding forming section by the application of mechanical pressure to the paper web.
It is known from the prior art to use one or more extended-nip presses in a press section that follows the forming section of a paper machine. In such an extended-nip press, the length of the pressing zone extending in the running direction of the paper web is substantially longer than that encountered in a roll press nip. Traditionally, extended-nip presses were commonly used only in paperboard machines and to make thicker grades of paper. However, more recently, extended-nip presses have also been introduced for use in the production of thinner paper grades, such as newspaper and fine papers.
Known press sections have various problems, especially when used in the high speed manufacture of relatively thin paper products. Relatively thin paper products are much more likely to be damaged during their manufacture than are thicker paper products, especially at relatively high web speeds of about 25 to about 35 m/sec. The relatively low number of intertwined fibers due to the very thinness of such thin paper products is one cause of such vulnerability. This latter factor also contributes to the difficulties encountered when attempting to separate such relatively thin paper stock from relatively smooth surfaces because considerable adhesion forces have to be overcome. Consequently, the incidence of tearing of and other damage to relatively thin paper webs is much higher even in such currently available press sections that employ extended nip presses than that encountered with thicker materials.
Examples of prior art that is considered to be most closely related to the present invention can be found in the following published patent documents: Finnish patent application No. 890530 (corresponding to U.S. Pat. No. 4,923,570 and published German patent document No. DE-OS 3808293.4), German utility model No. DE-GBM 8805966, Finnish patent application No. 913886 (corresponding to U.S. patent No. 5,178,732 and published German patent document No. DE-OS 4026021), Finnish patent No. 75,382, Finnish patent application No. 811403 (corresponding to U.S. Pat. No. 4,440,598), and U.S. Pat. Nos. 4,257,844, 4,551,205, 4,704,192, and 5,120,399. Further, reference is made to the paper in the journal Wochenblatt fxc3xcr Papierfabrikation 19 (1993), pages 780 to 782 xe2x80x9cDie Flexonip(copyright)-Pressenxe2x80x9d, which describes some of the latest extended nip press constructions of J. M. Voith GmbH. The press sections in these prior art devices have various problems.
In Finnish patent application No. 890,530, especially with reference to FIGS. 3 and 4 (and in the corresponding U.S. Pat. No. 4,923,570), the paper web is transferred from one extended nip press into a second and/or third extended nip press on the smooth surface of a gliding belt mantle of the extended nip press. Because the gliding belt mantle is smooth, water is pressed out of the paper web only through the surface of the paper web which faces away from the mantle because the smooth surface of the mantle has no route by which water can leave the paper web. Additionally, such a smooth surface mantle has pronounced adhesion properties and therefore poor paper web transfer properties. Moreover, the first nip is not an extended nip that removes water efficiently through both surfaces of the paper web, and the overall concept disclosed is not that of a compact press section of several nips.
In the FIGS. 1 and 2 embodiments in German utility model No. DE-GMB 88 05 966, the press section consists solely of two separate extended-nip presses in which the high load pressing is carried out on the same face of the same pressing felt. Moreover, the paper web is transferred over the long distance between the two nips by confining the paper web between two wet felts and on the face of one felt, thus resulting in the rewetting of the web by water previously expelled from the paper web to the felt. Consequently, the water content of the paper web increases after it is pressed in this so-called rewetting process. While the geometries of the press sections that are shown in FIGS. 3, 5 and 6 are more compact than those of FIGS. 1 and 2, the same press felt runs through both of the extended nips, thereby reducing the water absorbing capacity of the felt in the second extended nip. The transfer of the paper web from one extended nip to the other relies fully on the differences in the surface characteristics of the pressing felts as there is no additional mechanism to ensure the transfer of the paper web.
In Finnish patent application No. 913886 (corresponding to U.S. Pat. No. 5,178,732), FIGS. 1, 1a, 2, 3, 3a and 3b, a transfer means, which has a dewatering capacity lower than that of a felt, runs through all the press nips. Since the largest amount of water is removed in the first nip, the dewatering in one direction (i.e. through one surface of the paper web) is limited by the transfer means contacting that surface. In FIGS. 2, 2a, 3, 3a and 3b, the transfer means in the first nip consists of a gliding belt which is impervious to water and smooth so that dewatering through one surface of the paper web is completely precluded.
Finnish patent application No. 811403 (corresponding to U.S. Pat. No. 4,440,598), Finnish patent No. 75,382, and U.S. Pat. No. 4,257,844 do not disclose or suggest the use of an extended nip press with a high dewatering capacity at the first nip or anywhere else, or a closed web transfer to the dryer section.
U.S. Pat. No. 5,120,399 merely discloses the use of a single felt extended nip at the first nip. In this case, water is removed from the paper web in one direction only. Also, the press sections comprise only two nips, and no compact multi-nip press section is suggested.
The present invention involves a press section of a paper making machine for dewatering a paper web formed in a preceding forming section. In the press section of the present invention, a pick-up felt is used to remove the paper web from the forming wire that transports the paper web through the forming section of the paper making machine. The pick-up felt is applied to the upper surface of the paper web and then a lower felt is applied to the lower surface of the paper web. The two felts are comprised of a fabric material which are preferably slightly heavier and thicker than normal so that the felts absorb a substantial amount of water from the paper web. The two felts may also or instead be water permeable. The paper web, sandwiched between the two felts, passes through a first press nip which is an extended nip in that the paper web is pressed along its width and, in particular, along a portion of the length of the paper web, i.e., in the direction of travel or running direction of the web, rather than along a line of contact as in a conventional press nip. In the first extended nip, water is pressed from the upper surface of the paper web and is absorbed by the pick-up felt, and water is pressed from the lower surface of the paper web and is absorbed by the lower felt. Thus, in the first extended nip, water is simultaneously and symmetrically removed from both the upper and lower surfaces of the paper web along an extended length of the paper web. Consequently, good symmetry of the structure of the paper web through its thickness is obtained also resulting in symmetry of the density and porosity of both surfaces of the paper web. In this first press nip of the press section, relatively high pressures (about 100 to about 1,400 kN/m) can be applied to the paper web because it is sandwiched between the two felts. As a result, a substantial amount of water can be removed from the paper web in the first extended nip, thereby increasing its dry solids content and its strength and reducing the possibility that the paper web will break or otherwise be damaged when being transferred from one felt to another, when being transported between press nips, or when being removed from a felt or roll, even at relatively high web speeds of about 25 to about 35 m/sec.
After the lower felt is separated from the paper web, the paper web, supported by the pick-up felt, travels to a first roll nip where a center roll with a smooth surface directly contacts the paper web and additional water is pressed from the paper web into the pick-up felt. Alternatively, this first roll nip may be an extended nip and may be formed on one of the rolls forming the first extended nip or on a separate roll.
A roll nip may also be positioned between the first extended nip and the first roll nip formed on the center roll so that the paper web, sandwiched between the pick-up felt and the lower felt passes through this roll nip before the lower felt is separated from the paper web.
Downstream of the first roll nip, the pick-up felt is separated from the paper web and a press felt is applied to the upper surface of the paper web that was previously in contact with the pick-up felt. The paper web, supported on its upper surface by the press felt and on its lower surface by the center roll, then passes through a second roll nip where additional water is removed from the paper web to be absorbed by the press felt which is also comprised of a fabric material that absorbs water. The second roll nip may alternatively be replaced by an extended nip.
Downstream of the second roll nip, the press felt is removed, leaving the paper web on the smooth surface of the center roll. The paper web is then transferred in a short open draw to the lower surface of a drying wire and then to the first drying or lead-in cylinder of the following dryer section of the paper making machine. Means, such as for example a transfer band loop, may be employed to assist in such transfer.
From the point that the paper web enters the press section of the present invention through the first extended nip, the first roll nip and the second roll nip, the paper web is always supported and there are no open draws, thereby providing reliable transfers of the paper web from the forming section of the paper making machine through these three press nips.
The center roll (and thus the first and second roll nips which employ the center roll) is elevated relative to the first extended nip so that the paper web changes its direction of travel by a substantial degree, i.e., by no less than about 45xc2x0. A means is provided for altering the course of travel of the paper web without an open draw between the first extended nip and the first roll nip. Because a substantial amount of water is removed from the paper web at the first extended web, thereby significantly increasing its dry solids content, the paper web has sufficient structural strength upon leaving the first extended nip that the running direction of the paper web can be changed significantly without risking a break in the paper web, even to the extent that the paper web travels substantially vertically when supported by a felt. Consequently, by positioning the center roll and its two roll nips at a higher elevation than the extended nip, rather than on substantially the same level, a compact press section results which occupies a sufficiently small amount of space in the longitudinal direction of the machine (i.e., in the direction of travel of the paper web) so that it can substitute for existing press sections during the rebuilding, upgrading or modernization of an existing paper making machine to increase its dewatering capacity and running speed. The press section of the present invention can easily fit into the space occupied by the existing paper making machinery consisting of only roll nips such as the Sym-Press II(copyright) press made by Valmet Corporation.
The first extended nip is preferably comprised of two rolls: a hose roll with a flexible mantle and a press roll with a rigid, solid mantle with a hollow face, such as a grooved face, for example. In the interior of the flexible mantle of the hose roll proximate the point where the flexible mantle is closest to the press roll is a press or loading shoe. The press shoe and the flexible mantle apply pressure to the lower felt and the paper web W in the first extended nip throughout the area or zone of the extended nip, both in the direction of progress or advance of the paper web and in a direction transverse to the direction of progress the paper web. The hose roll and the press roll are stacked one above the other with either roll being in the upper position.
Steam boxes or other means to heat the paper web are preferably positioned in the press section of the present invention along the route of travel of the paper web to further increase the amount of water that is removed from the paper web in the press section. Additionally, vacuum boxes and suction zones in various rolls are preferably strategically positioned to ensure reliable transfer of the paper web between the various felts and rolls.
The press section of the present invention is particularly suited for manufacturing printing paper grades, such as newspaper or newsprint, SC-paper, LWC base paper, and fine paper. Additionally, the press section of this present invention can also be used for manufacturing paperboard, such as liner board or corrugated medium.
In an alternative embodiment of the press section of the present invention for use, for example, in the production of thicker paper grades or for operating the press section at above-average paper web running speeds, a second extended nip is interposed between the second roll nip and the drying section (which of course is downstream of the press section) to remove further water from the paper web before the paper web enters the drying section of the paper making machine. At this second extended nip, a lower felt can be applied to the lower surface of the paper web to aid in dewatering the paper web. When such a lower press felt is used, water is removed from the paper web in the second extended nip through the opposite surface (i.e., the lower surface) of the paper web that water was removed through the first and second roll nips (i.e., the upper surface), thereby further contributing to the symmetry of the structure of the paper web through its thickness. The second extended nip has a construction that is similar to that of the first extended nip. Additionally, an upper press felt can be applied to the upper surface of the paper web as the paper web passes through the second extended nip. Alternatively, instead of the second extended nip, another roll nip may be substituted.
In an alternative embodiment, the upper roll of the first extended nip may also constitute one of the rolls of the first roll nip. Furthermore, more than two roll nips may be situated around the center roll.
As previously noted, to further ensure that the paper web separates from the center roll intact, a transfer band loop may encircle the center roll so that the paper web does not contact the center roll. The transfer band loop is preferably made of fabric and the surface thereof that contacts the paper web is preferably smooth. The transfer band loop may also extend through the second extended nip if it is also included in the press section, and to the drying wire that leads to first drying or lead-in cylinder of the following dryer section of the paper making machine so that there are no open draws in the entire press section.
In another embodiment of the present invention, a primary roll or extended nip is used to remove water from the paper web while the paper web is still supported by the forming wire that transfers the paper web from the forming section to the press section. A press felt is applied to the upper surface of the paper web before the paper web supported on its lower surface by the forming wire enters this primary roll or extended nip. Because of the high water content of the paper web at this point, the press felt is preferably a relatively water permeable and open fabric that also absorbs water. The primary press nip is particularly useful in the manufacture of paperboard or other paper that is thicker than average, in paper making machines employing pulp grades which are relatively difficult to dewater, or in paper making machines where the desired paper web running speed is very high.
Other objects and features of the present invention will become apparent from the following detailed description considered in conjunction with the accompanying drawings. It is to be understood, however, that the drawings are intended solely for purposes of illustration and not as a definition of the limits of the invention, for which reference should be made to the appended claims.